Conventional circuit arrangements and methods of the above-mentioned general type are known. In fact, this type of frequency generation is typical in practically all present-day circuit arrangements for generating a frequency modulated transmission signal with several channels. The oscillator provides a carrier frequency that is modulated by an information signal. In this context, the modulation swing or range of the modulation of the signal refers to the deviation of the frequency of the modulated signal relative to the carrier frequency.
The d.c. signal for controlling the oscillator can be a d.c. control voltage or a d.c. control current. The frequency-determining components of the network of the oscillator may comprise especially capacitances and/or inductances, i.e. physically capacitors and/or inductors. A phase regulating loop, and particularly a phase-locked loop (PLL), used in such a circuit arrangement typically consists of the oscillator, a phase/frequency detector to which the signal of the oscillator and a reference frequency are provided, and a loop filter that filters the output signal of the phase/frequency detector and forms thereof a d.c. signal as a control signal for the oscillator. In the circuit loop, the components cooperate in such a manner so that the oscillator frequency approaches and approximates the reference frequency.
In principle, frequency modulated transmitters can be driven or operated over a wide range of various differently-dimensioned frequency swings. However, the signal-to-noise ratio of the demodulated signal becomes ever worse as the frequency swing gets smaller. That means that the transmission range diminishes for the same transmitting power, or viewed alternatively, more transmitting power is required to achieve the same transmission range. On the other hand, as the frequency swing gets larger, the ratio between the transmission range or distance and the transmitting power becomes evermore advantageous. However, in this context, simultaneously the spectral bandwidth occupied by the transmitter increases sharply, especially above the transition from narrow band to wide band frequency modulation (FM). As a result, the arising adjacent channel interference becomes unacceptably strong, or the interchannel spacing or separation must be substantially increased in view of these effects, so that as an ultimate consequence, the number of useable channels in the allocated frequency band is sharply reduced.
In view of the above effects and the results thereof, the ability to maintain the modulation swing as exactly as possible is a very important quality criterium for evaluating every frequency modulating transmitting arrangement. Moreover, the maximum and minimum limits or boundaries of the permissible modulation swings are very often prescribed or regulated in the applicable industry standards, for example especially in connection with the present-day typical digital modulation processes such as frequency shift keying (FSK) and its variants.
In connection with the above considerations, it is generally known that the fabrication tolerances in the production of the components such as capacitors, coils, varactor diodes, etc. used in such circuit arrangements generally are not adequate in order to ensure a sufficient accuracy of the modulation swing. In the past, it was typical to provide for the adjustment of the modulation swing by manual trimming of the circuit arrangement through the use of a trimming potentiometer. Today it is typical to install digitally adjustable amplifying elements or damping elements at suitable locations within the modulation signal path for carrying out the required adjustment, or to digitally adjust the amplitude of the modulation signal already in the generation of the signal. In this context, the adjustment value is typically stored in an EEPROM. In other words, the scatter or spread of the modulation swing caused by production tolerances and the like has typically been reduced by additional adjustment or balancing, either by hand or by digitally storing adjustment values in connection with the final testing of the device, so that the required device specifications would be met and maintained by new devices.
It is disadvantageous in the above described conventional approaches to achieve the required modulation swing, that the manual adjusting or the testing and storing of digital adjustment values necessitate additional work steps in the production of the device, which lead to additional costs of the device. Furthermore, the parameters adjusted to achieve the required modulation swing of the device at the end of the manufacturing process can never match the true optimal values for the actual real world application or use of the device in the field. For example, this is true already because reserves must be provided, or an over-adjustment must be carried out, in order to allow for long time drift, temperature dependence, supply voltage dependence, dependence on the actual present selected channel frequency, and the like in the actual utilization of the device. Even if such factors are taken into account, there can be no absolute assurance that the assumed drift values and the like will actually apply to a particular sample of a device. In other words a particular sample of the device may significantly overshoot or undershoot the expected nominal drift behavior.